Thermal dye transfer methods include thermal dye sublimation transfer also called thermal dye diffusion transfer. This is a recording method in which a dye-donor element provided with a dye layer containing sublimating dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye is transferred from the selectively heated regions of the dye-donor element to the receiver sheet and forms a pattern thereon, the shape and density of which are in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer. Normally, the opposite side of the support is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
The dye layer can be a monochromic dye layer or it may comprise sequential repeating areas of differently coloured dyes e.g. dyes having a cyan, magenta, yellow, and optionally black hue. When a dye-donor element containing three or more primary colour dyes is used, a multicolour image can be obtained by sequentially performing the dye transfer process steps for each colour.
Many of the dyes proposed for use in thermal dye sublimation transfer are not sufficient in performance since they yield inadequate transfer densities at reasonable coating coverages, or because they have inadequate spectral characteristics for substractive colour systems, or because they have a poor light-fastness.
Yellow dyes that satisfy the absorption requirements can be found in the class of the so-called dicyanovinylaniline dyes. Such yellow dyes have been described for use in thermal transfer recording in U.S. Pat. No. 4,833,123, EP 271,861, and in JP 84/78,895.
Although many yellow dyes of this class have indeed excellent spectral absorption characteristics, they lack sufficient light-fastness.
The poor light-fastness of dicyanovinylaniline dyes used for thermal printing has been emphasized in Chemistry and Industry, 16 Oct. 1989 page 681, FIG. 5.